1. Field of the Invention
The present invention relates to the production of trialkoxysilanes by the Direct Synthesis of silicon with alcohols in the presence of a copper catalyst. This Direct Synthesis exhibits short induction times, high selectivity for trialkoxysilanes, high overall silicon conversion, and high, stable reaction rates.
2. Description of Related Art
Trialkoxysilanes, especially trimethoxysilane and triethoxysilane, are used in the production of silane coupling agents. One method of synthesizing trialkoxysilanes is directly from silicon and an alcohol in the presence of copper or a copper compound. This method is known variously in the art as the “Direct Synthesis”, the “Direct Reaction,” the “Direct Process,” or the Rochow Reaction. For trialkoxysilanes, it is most conveniently performed in slurry reactors.
U.S. Pat. No. 3,641,077 to Rochow which issued on Feb. 8, 1972, discloses the Direct Synthesis of trialkoxysilanes in a slurry reactor using catalytically activated silicon particles suspended in a thermally stable, high boiling solvent reacted with an alcohol at an elevated temperature. Trialkoxysilanes were prepared by directly reacting a copper-silicon mass, suspended in a silicone oil, with alcohol at 250 to 300° C. The copper-silicon mass contained about 10 wt. % copper and was prepared by heating copper and silicon at temperatures above 1000° C. in a furnace in the presence of a hydrogen gas stream. Low yields of trialkoxysilanes are generally obtained using this method.
U.S. Pat. No. 3,775,457 to Muraoka et al. which issued on Nov. 27, 1973, teaches the use of polyaromatic hydrocarbon oils as solvents in the Direct Synthesis of trialkoxysilanes from an alcohol and finely divided silicon metal activated with cuprous chloride catalyst. The use of cuprous chloride provides an increased yield over that obtained using the sintered copper-silicon mass taught in U.S. Pat. No. 3,641,077 to Rochow.
The use of cuprous chloride or cupric chloride with alkylated benzene solvents such as dodecylbenzene and tridecylbenzene as disclosed in U.S. Pat. No. 5,362,897 to Harada et al. which issued on Nov. 8, 1994, Japanese Kokai Patent Application 55-28928 (1980), 55-28929 (1980), 55-76891 (1980), 57-108094 (1982) and 62-96433 (1987), also affords increased yields of trialkoxysilanes. It is advantageous to use the alkylated benzene solvents because they are less expensive and less hazardous to people and the environment than the polyaromatic hydrocarbon solvents taught in U.S. Pat. No. 3,775,457.
U.S. Pat. No. 5,362,897 to Harada et al., claims the use of specially prepared “wet process” cuprous chloride, CuCl, in preference to commercial “dry process” cuprous chloride, to afford higher reaction rate and silicon conversion. “Wet process” cuprous chloride is defined therein as that “prepared through the steps of crystallization and separation and drying.” “Dry process” cuprous chloride is prepared from metallic copper and chlorine gas. Preferably, the “wet process” cuprous chloride is less than 2 μm in size.
Japanese Kokai Patent Application 11-21288 (1999) discloses the use of wet or dry cuprous chloride to activate silicon metal by heating at 250° C. in a straight chain alkylated benzene solvent for at least 3 hours. The particle size of the cuprous chloride was 0.1 to 50 μm, preferably 0.5 to 10 μm.
The use of copper (II) hydroxide as a catalyst is disclosed in U.S. Pat. No. 4,727,173 to Medicino which issued on Feb. 23, 1988. Limitations associated with cuprous chloride were avoided and high selectivity to trialkoxysilanes was reported. The preferred solvents were diphenyl ether, polyaromatic hydrocarbons like THERMINOL® 59, THERMINOL® 60, THERMINOL® 66, and alkylated benzenes such as dodecylbenzene. However, U.S. Pat. No. 5,728,858 to Lewis et al. which issued on Mar. 17, 1998, discloses that when copper (II) hydroxide is used in combination with alkylated benzene solvents such as dodecylbenzene, the Direct Synthesis becomes unstable after about 25 to 35 wt. % silicon has reacted. When methanol is the alcohol reactant at temperatures over 220° C., trimethoxysilane content in the reaction product declines from approximately 90 to 95 wt. % to 50 to 60 wt. %. After 60 wt. % silicon conversion, the trimethoxysilane content increases to 80 to 95 wt. %. Simultaneous with the loss of selectivity is the enhanced formation of methane, water, and dimethyl ether. Methane and dimethyl ether formation result from the inefficient use of methanol. Water reacts with trialkoxysilanes to produce soluble, gelled and/or resinous organic silicates which cause foaming leading to incomplete recovery of the reaction solvent.
U.S. Pat. No. 5,728,858 to Lewis et al. also teaches the reductive activation of copper (II) hydroxide/silicon slurries with hydrogen gas, carbon monoxide, monosilane or polyaromatic hydrocarbons to obtain active, selective, and stable Direct Synthesis of trialkoxysilanes in alkylated benzene solvents such as the linear alkylate NALKYLENE® 550BL. Particle size of the copper (II) hydroxide is desirably 0.1 to 50 μm, and preferably 0.1 to 30 μm.
The use of hydrogen to activate silicon with copper for the Direct Synthesis is well known in the prior art. Hydrogen activation is accomplished at temperatures above 400° C. in fixed bed reactors, fluidized bed reactors or furnaces with silicon-copper catalyst mixtures containing more than 1.5 wt. % copper. However, the prior art provides little information regarding selectivity, reactivity, and reaction stability of the silicon-copper masses in slurry phase Direct Synthesis of trialkoxysilanes.
In Suzuki et al., Bulletin of the Chemical Society of Japan, Vol. 1 (1994) pp. 3445-3447, the hydrogen activation of silicon and cupric chloride mixtures having 2.5 wt. % copper in a fixed bed reactor at 260° C. afforded complete silicon conversion and 89% selectivity to trimethoxysilane in a Direct Synthesis with methanol. The duration of the induction period, reaction rate, and selectivity to the trimethoxysilane were all very dependent on the temperature of the hydrogen activation.
The use of other copper catalysts such as copper alkoxides, with or without copper chlorides, and cupric oxide are also taught in the prior art. However, the prior art does not mention any particular particle size of these copper catalysts.
Alcohol dehydration and dehydrogenation are especially troublesome problems when ethanol and other higher homologs are used in the Direct Synthesis. At temperatures greater than 250° C., alkenes, aldehydes and acetals, and not the desired trialkoxysilanes, are formed in significant amounts. Even when these are not the predominant products, their presence in the reaction mixture may inhibit further catalytic activity. At lower temperatures, (for example 220° C.), alcohol decomposition reactions are less prevalent, but the Direct Synthesis is slower. Japanese Patent Application Kokai 55-2641 (1980) discloses the use of cyclic ethers to improve reactivity and selectivity to triethoxysilane when the Direct Synthesis is conducted in dodecylbenzene at these lower temperatures. However, cyclic ethers such as dibenzo-18-crown-6 are quite expensive; others such as 12-crown-4 are also toxic.
In spite of the improvements and advances taught in the prior art, there continues to exist the need for a stable, highly selective and rapid Direct Synthesis of trialkoxysilanes which produces less waste and avoids the deficiencies of conventionally prepared copper chlorides, alkylated benzene solvents and specially selected silicon samples. In particular, there is a need for such a Direct Synthesis, which eliminates or avoids the alcohol reduction, alcohol dehydrogenation and alcohol dehydration side reactions typical of ethanol and the higher alcohols. There is also a need for a Direct Synthesis of trialkoxysilane which is desirably and acceptably reactive, selective and stable with silicon samples spanning a wide range of manufacturing methods, trace metal concentrations, silicide intermetallic phases, oxygen contents and surface oxidation.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a Direct Synthesis of trialkoxysilanes from silicon metal and alcohol that has enhanced selectivity to trialkoxysilane over tetraalkoxysilane throughout the entire course of the reaction having high silicon conversion and stable reaction rate, and the resultant trialkoxysilane.
It is another object of the present invention to provide a Direct Synthesis of trialkoxysilanes which produces less waste products and avoids the inefficient use of the alcohol reactant by eliminating or avoiding the alcohol reduction, alcohol dehydrogenation and alcohol dehydration side reactions when ethanol and higher alcohols are used.
A further object of the invention is to provide a Direct Synthesis of trialkoxysilanes that uses more desirable solvents.
It is yet another object of the present invention to provide a Direct Synthesis of trimethoxysilane and triethoxysilane which is more economically and environmentally viable.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.